Liquid crystal medium and liquid crystal device

ABSTRACT

The invention relates to a medium comprising at least 60% of one or more compounds of formula I 
     
       
         
         
             
             
         
       
     
     wherein R 11 , R 12 , MG 11 , MG 12  and Sp 1  have the meaning given herein below, to the use of such media in liquid crystal devices, in particular in flexoelectric liquid crystal devices, and to a flexoelectric liquid crystal device comprising a liquid crystal medium according to the present invention.

The invention relates to a medium comprising at least 60% of one or morecompounds of formula I

wherein R¹, R¹², MG¹¹, MG¹² and Sp¹ have the meaning given herein below,to the use of such media in liquid crystal devices, in particular inflexoelectric liquid crystal devices, and to a flexoelectric liquidcrystal device comprising a liquid crystal medium according to thepresent invention.

Liquid Crystal Displays (LCDs) are widely used to display information.LCDs are used for direct view displays, as well as for projection typedisplays. The electro-optical mode which is employed for most displaysstill is the twisted nematic (TN)-mode with its various modifications.Besides this mode, the super twisted nematic (STN)-mode and morerecently the optically compensated bend (OCB)-mode and the electricallycontrolled birefringence (ECB)-mode with their various modifications, ase. g. the vertically aligned nematic (VAN), the patterned ITO verticallyaligned nematic (PVA)-, the polymer stabilized vertically alignednematic (PSVA)-mode and the multi domain vertically aligned nematic(MVA)-mode, as well as others, have been increasingly used. All thesemodes use an electrical field, which is substantially perpendicular tothe substrates, respectively to the liquid crystal layer. Besides thesemodes there are also electro-optical modes employing an electrical fieldsubstantially parallel to the substrates, respectively the liquidcrystal layer, like e.g. the In Plane Switching (short IPS) mode (asdisclosed e.g. in DE 40 00 451 and EP 0 588 568) and the Fringe FieldSwitching (FFS) mode. Especially the latter mentioned electro-opticalmodes, which have good viewing angle properties and improved responsetimes, are increasingly used for LCDs for modern desktop monitors andeven for displays for TV and for multimedia applications and thus arecompeting with the TN-LCDs.

Further to these displays, new display modes using cholesteric liquidcrystals having a relatively short cholesteric pitch have been proposedfor use in displays exploiting the so called “flexo-electric” effect.The term “liquid crystal”, “mesomorphic compound” or “mesogeniccompound” (also shortly referred to as “mesogen”) means a compound thatunder suitable conditions of temperature, pressure and concentration canexist as a mesophase (nematic, smectic, etc.) or in particular as a LCphase. Non-amphiphilic mesogenic compounds comprise for example one ormore calamitic, banana-shaped or discotic mesogenic groups.

Flexoelectric liquid crystal materials are known in prior art. Theflexoelectric effect is described inter alia by Chandrasekhar, “LiquidCrystals”, 2nd edition, Cambridge University Press (1992) and P. G.deGennes et al., “The Physics of Liquid Crystals”, 2nd edition, OxfordScience Publications (1995).

In these displays, the cholesteric liquid crystals are oriented in the“uniformly lying helix” arrangement (ULH), which also give this displaymode its name. For this purpose, a chiral substance, which is mixed witha nematic material, induces a helical twist transforming the materialinto a chiral nematic material, which is equivalent to a cholestericmaterial. The term “chiral” in general is used to describe an objectthat is non-superimposable on its mirror image. “Achiral” (non-chiral)objects are objects that are identical to their mirror image. The termschiral nematic and cholesteric are used synonymously in thisapplication, unless explicitly stated otherwise. The pitch induced bythe chiral substance (P₀) is in a first approximation inverselyproportional to the concentration (c) of the chiral material used. Theconstant of proportionality of this relation is called the helicaltwisting power (HTP) of the chiral substance and defined by equation (1)

HTP≡1/(c·P ₀)  (1)

wherein

c is concentration of the chiral compound.

The uniform lying helix texture is realized using a chiral nematicliquid crystal with a short pitch, typically in the range from 0.2 μm to1 μm, preferably of 1.0 μm or less, in particular of 0.5 μm or less,which is unidirectional aligned with its helical axis parallel to thesubstrates, e. g. glass plates, of a liquid crystal cell. In thisconfiguration, the helical axis of the chiral nematic liquid crystal isequivalent to the optical axis of a birefringent plate.

If an electrical field is applied to this configuration normal to thehelical axis the optical axis is rotated in the plane of the cell,similar as the director of a ferroelectric liquid crystal rotate as in asurface stabilized ferroelectric liquid crystal display. Theflexoelectric effect is characterized by fast response times typicallyranging from 6 μs to 100 μs. It further features excellent grey scalecapability.

The field induces a splay bend structure in the director, which isaccommodated by a tilt in the optical axis. The angle of the rotation ofthe axis is in first approximation directly and linearly proportional tothe strength of the electrical field. The optical effect can beobserved, when the liquid crystal cell is placed between crossedpolarizers with the optical axis in the unpowered state at an angle of22.5° to the absorption axis of one of the polarizers. This angle of22.5° is also the ideal angle of rotation of the electric field, asthus, by the inversion the electrical field, the optical axis is rotatedby 45° and by appropriate selection of the relative orientations of thepreferred direction of the axis of the helix, the absorption axis of thepolarizer and the direction of the electric field, the optical axis canbe switched from parallel to one polarizer to the center angle betweenboth polarizers. The optimum contrast is then achieved when the totalangle of the switching of the optical axis is 45°. In that case thearrangement can be used as a switchable quarter wave plate, provided theoptical retardation, i. e. the product of the effective birefringence ofthe liquid crystal and the cell gap, is selected to be the quarter ofthe wave length. In this context, the wavelength referred to is 550 nm,the wavelength for which the sensitivity of the human eye is highest,unless explicitly stated otherwise.

The angle of rotation of the optical axis (Φ) is given in goodapproximation by formula (2)

tan Φ=ēP ₀ E/(2πK)  (2)

wherein

-   P₀ is the undisturbed pitch of the cholesteric liquid crystal,-   ē is the average [ē=½ (e_(splay)+e_(bend))] of the splay    flexoelectric coefficient (e_(splay)) and the bend flexoelectric    coefficient (e_(bend)),-   E is the electrical field strength and-   K is the average [K=½ (k₁₁+k₃₃)] of the splay elastic constant (k₁₁)    and the bend elastic constant (K₃₃)

and wherein

-   ē/K is called the flexo-elastic ratio.

This angle of rotation is half the switching angle in a flexoelectricswitching element.

The response time (τ) of this electro-optical effect is given in goodapproximation by formula (3)

τ=[P ₀/(2π)]² ·γ/K  (3)

wherein

-   γ is the effective viscosity coefficient associated with the    distortion of the helix.

There is a critical field (E_(c)) to unwind the helix, which can beobtained from equation (4)

E _(c)=(π² /P ₀)·[k ₂₂/(ε₀·Δε)]^(1/2)  (4)

wherein

k₂₂ is the twist elastic constant,

ε₀ is the permittivity of vacuum and

Δε is the dielectric anisotropy of the liquid crystal.

In this mode, however several problems still have to be resolved, whichare, amongst others, difficulties in obtaining the required uniformorientation, an unfavourably high voltage required for addressing, whichis incompatible with common driving electronics, a not really dark “offstate”, which deteriorates the contrast, and, last not least, apronounced hysteresis in the electro-optical characteristics.

A relatively new display mode, the so-called uniformly standing helix(USH) mode, may be considered as an alternative mode to succeed the IPS,as it can show improved black levels, even compared to other displaymode providing wide viewing angles (e.g. IPS, VA etc.).

For the USH mode, like for the ULH mode, flexoelectric switching hasbeen proposed, using bimesogenic liquid crystal materials. Bimesogeniccompounds are known in general from prior art (cf. also Hori, K.,limuro, M., Nakao, A., Toriumi, H., J. Mol. Struc. 2004, 699, 23-29).The term “bimesogenic compound” relates to compounds comprising twomesogenic groups in the molecule. Just like normal mesogens they canform many mesophases, depending on their structure. In particular,compounds of formula I induce a second nematic phase, when added to anematic liquid crystal medium.

The term “mesogenic group” means in this context, a group with theability to induce liquid crystal (LC) phase behaviour. The compoundscomprising mesogenic groups do not necessarily have to exhibit an LCphase themselves. It is also possible that they show LC phase behaviouronly in mixtures with other compounds. For the sake of simplicity, theterm “liquid crystal” is used hereinafter for both mesogenic and LCmaterials.

However, due to the unfavourably high driving voltage required, to therelatively narrow phase range of the chiral nematic materials and totheir irreversible switching properties, materials from prior art arenot compatible for the use with current LCD driving schemes.

For displays of the USH and ULH mode, new liquid crystalline media withimproved properties are required. Especially the birefringence (Δn)should be optimized for the optical mode. The birefringence Δn herein isdefined in equation (5)

Δn=n _(e) −n _(o)  (5)

wherein n_(e) is the extraordinary refractive index and n_(o) is theordinary refractive index, and the average refractive index n_(av.) isgiven by the following equation (6).

n _(av.)=[(2n _(o) ² +n _(e) ²)/3]^(1/2)  (6)

The extraordinary refractive index n_(e) and the ordinary refractiveindex n_(o) can be measured using an Abbe refractometer. An can then becalculated from equation (5).

Furthermore, for displays utilizing the USH or ULH mode the opticalretardation d*Δn (effective) of the liquid crystal media shouldpreferably be such that the equation (7)

sin²(π·d·Δn/λ)=1  (7)

wherein

d is the cell gap and

λ is the wave length of light

is satisfied. The allowance of deviation for the right hand side ofequation (7) is +/−3%.

The wavelength of light generally referred to in this application is 550nm, unless explicitly specified otherwise.

The cell gap of the cells preferably is in the range from 1 μm to 20 μm,in particular within the range from 2.0 μm to 10 μm.

For the ULH/USH mode, the dielectric anisotropy (Δε) should be as smallas possible, to prevent unwinding of the helix upon application of theaddressing voltage. Preferably Δε should be slightly higher than 0 andvery preferably be 0.1 or more, but preferably 10 or less, morepreferably 7 or less and most preferably 5 or less. In the presentapplication the term “dielectrically positive” is used for compounds orcomponents with Δε>3.0, “dielectrically neutral” with −1.5≤Δε≤3.0 and“dielectrically negative” with Δε<−1.5. Δε is determined at a frequencyof 1 kHz and at 20° C. The dielectric anisotropy of the respectivecompound is determined from the results of a solution of 10% of therespective individual compound in a nematic host mixture. In case thesolubility of the respective compound in the host medium is less than10% its concentration is reduced by a factor of 2 until the resultantmedium is stable enough at least to allow the determination of itsproperties. Preferably, the concentration is kept at least at 5%,however, in order to keep the significance of the results a high aspossible. The capacitance of the test mixtures are determined both in acell with homeotropic and with homogeneous alignment. The cell gap ofboth types of cells is approximately 20 μm. The voltage applied is arectangular wave with a frequency of 1 kHz and a root mean square valuetypically of 0.5 V to 1.0 V; however, it is always selected to be belowthe capacitive threshold of the respective test mixture.

Δε is defined as (ε∥−ε_(⊥)), whereas ε_(av.) is (ε∥+2 ε_(⊥))/3. Thedielectric permittivity of the compounds is determined from the changeof the respective values of a host medium upon addition of the compoundsof interest. The values are extrapolated to a concentration of thecompounds of interest of 100%. A typical host medium is ZLI-4792 orBL-087 both commercially available from Merck, Darmstadt.

Besides the above-mentioned parameters, the media have to exhibit asuitably wide range of the nematic phase, a rather small rotationalviscosity and an at least moderately high specific resistivity.

Similar liquid crystal compositions with short cholesteric pitch forflexoelectric devices are known from EP 0 971 016, GB 2 356 629 andColes, H. J., Musgrave, B., Coles, M. J., and Willmott, J., J. Mater.Chem., 11, p. 2709-2716 (2001). EP 0 971 016 reports on mesogenicestradiols, which, as such, have a high flexoelectric coefficient. GB 2356 629 discloses broad generic formulae of bimesogenic compounds andsuggests the use of these bimesogenic compounds in flexoelectricdevices. The flexoelectric effect herein has been investigated in purecholesteric liquid crystal compounds and in mixtures of homologouscompounds only so far. Most of these compounds were used in binarymixtures consisting of a chiral additive and a nematic liquid crystalmaterial being either a simple, conventional mono-mesogenic material ora bimesogenic one. These materials do have several drawbacks forpractical applications, like insufficiently wide temperature ranges ofthe chiral nematic— or cholesteric phase, too small flexoelectricratios, small angles of rotation.

Non-symmetrically linked bimesogenic compounds are proposed e.g. in WO2014/005672 (A).

EP 0 233 688 discloses bis(phenylethanolamines) andbis(phenoxypropanol-amines) useful as beta-agonists.

Macro Rings. 11. Polynuclear Paracyclophanes' H. Steinberg, Donald JCram, JACS, (1952), 74, p. 5388-91 discloses some compounds.

One aim of the invention is to provide improved flexoelectric devicesthat exhibit high switching angles and fast response times. Another aimwas to provide liquid crystal materials with advantageous properties, inparticular for use in flexoelectric displays that enable good uniformalignment over the entire area of the display cell without the use of amechanical shearing process, good contrast, high switching angles andfast response times also at low temperatures. The liquid crystalmaterials should exhibit low melting points, broad chiral nematic phaseranges, short temperature independent pitch lengths and highflexoelectric coefficients. Other aims of the present invention areimmediately evident to the person skilled in the art from the followingdetailed description.

Using one terminal alkyl group and keeping the other as adipole-inducing group shows a significant effect in improving thealignment of the mixture and maintaining the good flexoelectricproperties of the molecule at the same time. Using materials, whichimprove the homeotropic alignment quality of a mixture, have shown animproved alignment when placed in the chiral Uniform Lying Helixorientation.

Flexoelectric bimesogenic materials should include a lateral dipole tohave the greatest electro optic flexoelectric response, based on themost widely accepted theories of flexoelectricity. Materials havingterminal alkyl chains lose the balanced high polarities, which helpprovide a dipole across the length of the molecule. Rather, thematerials now have a terminal dipole, materials of this terminal alkyltype are also shown to improve the alignment of the liquid crystalsystem in which they are used.

The inventors have found out that the one or more of the above-mentionedaims can be surprisingly achieved by providing a medium according to thepresent invention.

Thus, the invention relates to a medium comprising at least 60% of oneor more compounds of formula I

wherein

-   R¹¹ and R¹² are each independently H, F, Cl, CN, NCS or a    straight-chain or branched alkyl group with 1 to 25 C atoms, which    may be unsubstituted, mono- or polysubstituted by halogen or CN, it    being also possible for one or more non-adjacent CH₂ groups to be    replaced, in each occurrence independently from one another, by —O—,    —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —O—CO—O—, —S—CO—, —CO—S—,    —CH═CH—, —CH═CF—, —CF═CF— or —C≡C— in such a manner that oxygen    atoms are not linked directly to one another, preferably F, Cl, CN,    a straight-chain or branched alkyl group with 1 to 25 C atoms which    may be unsubstituted, mono- or polysubstituted by halogen or CN more    preferably a polar group, most preferably F, CN, CF₃,

at least one of

-   R¹¹ and R¹² is an alkyl group, i.e. a straight-chain or branched    alkyl group with 1 to 25 C atoms, which may be unsubstituted, mono-    or polysubstituted by halogen or CN, it being also possible for one    or more non-adjacent CH₂ groups to be replaced, in each occurrence    independently from one another, by —O—, —S—, —NH—, —N(CH₃)—, —CO—,    —COO—, —OCO—, —O—CO—O—, —S—CO—, —CO—S—, —CH═CH—, —CH═CF—, —CF═CF— or    —C≡C— in such a manner that oxygen atoms are not linked directly to    one another, preferably a polar group, in which one CH₂ groups is    replaced by —CH═CH—, —CH═CF—, —CF═CF—, but from which OCF₃ and CF₃,    are excluded, preferably a non-polar group, more preferably    unsubstituted alkyl, alkenyl or alkinyl, most preferably a    straight-chain or branched alkyl group with 1 to 25 C atoms,-   MG¹¹ and MG¹² are each independently a mesogenic group,-   at least one of-   MG¹¹ and MG¹² comprises one, two or more 5-atomic and/or 6-atomic    rings, in case of comprising two or more 5- and/or 6-atomic rings at    least two of these may be linked by a 2-atomic linking group,    preferably selected from the group of linking groups —CO—O—, —O—CO—,    —CH₂—O—, —O—CH₂—, —CF₂—O— and —O—CF₂—,-   Sp¹ is a spacer group comprising 1, 3 or 5 to 40 C atoms, wherein    one or more non-adjacent and non-terminal CH₂ groups may also be    replaced by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—, —S—CO—, —O—COO—,    —CO—S—, —CO—O—, —CH(halogen)-, —CH(CN)—, —CH═CH— or —C≡C—, however    in such a way that no two O-atoms are adjacent to one another, now    two —CH═CH— groups are adjacent to each other and no two groups    selected from —O—CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O— and —CH═CH—    are adjacent to each other, preferably —CH₂)_(n)— (i.e. 1,n-alkylene    with n C atoms), with n an integer, preferably from 3 to 19, more    preferably from 3 to 11, most preferably an odd integer (i.e. 3, 5,    7, 9 or 11),-   X¹¹ and X¹² are independently from one another a linking group    selected from —CO—O—, —O—CO—, —O—, —CH═CH—, —C≡C—, —CO—S—, —S—CO—,    —CS—S—, —S—, —CF₂—O—, —O—CF₂—, —CF₂—CF₂—, —CH₂—O—, —O—CH₂— and a    single bond, preferably they are —CO—O—, —O—CO—, —CF₂—O—, OCF₂ or a    single bond, most preferably a single bond or —CF₂—O—, —O—CF₂—,    however under the condition that in —X¹-Sp¹-X¹²— no two O-atoms are    adjacent to one another, no two —CH═CH— groups are adjacent to each    other and no two groups selected from —O—CO—, —S—CO—, —O—COO—,    —CO—S—, —CO—O— and —CH═CH— are adjacent to each other.

In a first preferred embodiment

-   R¹¹ is a straight-chain or branched alkyl group with 1 to 25 C    atoms, preferably 1 to 15 C atoms, more preferably 1 to 10 C atoms    and-   R¹² is H, F, Cl, CN, or a straight-chain or branched alkyl group    with 1 to 25 C atoms which may be unsubstituted, mono- or    polysubstituted by halogen or CN, preferably CN and/or-   MG¹¹ and/or MG¹² is a mesogenic group, which comprises one or more    6-atomic rings, optionally substituted by F,Cl,CN, OCH₃, OCF₃ at    least two of these may be linked by a 2-atomic linking group,    preferably selected from the group of linking groups —CO—O—, —O—CO—,    —CH₂—O—, —O—CH₂—, —CF₂—O— and —O—CF₂—, and/or-   Sp¹ is a spacer group comprising 5 to 40 C atoms, wherein one or    more non-adjacent and non-terminal CH₂ groups may also be replaced    by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—, —S—CO—, —O—COO—, —CO—S—,    —CO—O—, —CH(halogen)-, —CH(CN)—, —CH═CH— or —C≡C— and/or-   X¹¹ and X¹² are each independently of one another a group selected    from —CH═CH—, —C≡C—, —O—, —CF₂—O—, —O—CF₂—, —CO—O—, —O—CO—,    —O—CO—O—, —S—, —CS— S—, —S—CS—, —CO—S—, —S—CO—, —S—CO—S— and    —S—CS—S— or a single bond, preferably selected from —O—, —CO—O—,    —O—CO—, —S—CO— and —CO—S— or a single bond, most preferably —CO—S—,    —S—CO—, —O—CO—, —CO—O— or a single bond, however under the condition    that in —X¹¹-Sp¹-X¹²— no two O-atoms are adjacent to one another,    now two —CH═CH— groups are adjacent to each other and no two groups    selected from —O—CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O—and —CH═CH— are    adjacent to each other.

In a further preferred embodiment, which may be different from oridentical to any of the preferred embodiments,

-   X¹¹ and X¹² are different from each another and otherwise    independently from one another are a linking group as defined above,    preferably selected from —CO—O—, —O—CO—, —CH═CH—, —C≡C—, —O—,    —S—CO—, —CO—S—, —S—, and —CO—, or a single bond, preferably X¹¹ is    —CO—O— or —O— and X¹² is a single bond or X¹¹ is —CO—O— and X¹² is    —O—, most preferably X¹¹ is —CO—O— and X¹² is —CO—O— or a single    bond.

In a further preferred embodiment, which may be different from oridentical to any of the preferred embodiments,

-   at least one of-   MG¹¹ and MG¹² comprises two or more 5- or 6-atomic rings, at least    two of which are linked by a 2-atomic linking group, preferably    selected from the group of linking groups —CO—O—, —O—CO—, —CH₂—O—,    —O—CH₂—, —CF₂—O— and —O—CF₂—.

In a further preferred embodiment, which may be different from oridentical with any of the preferred embodiments,

-   X¹¹ is a group selected from —CO—S—, —S—CO—, —CS—S—, —S—CS—, —CO—S—,    —S—CO—, —S—CO—S—, —S—CS—S— and —S—, preferably —CO—S—, —S—CO— or    —S—, most preferably —CO—S— and —S—CO—,-   X¹² independently from X¹¹ has one of the meanings given for X¹¹ or    is a linking group selected from —CO—O—, —O—CO—, —CH═CH—, —C≡C—,    —CF₂—O— or —O—CF₂— and —O— or a single bond, preferably it is —CO—S—    or —S—, most preferably —CF₂—O— or —O—CF₂—,-   Sp¹ is —(CH₂)_(n)— with-   n 1, 3 or an integer from 5 to 15, most preferably an odd (i.e.    uneven) integer and, most preferably 5, 7 or 9, but may be an even    integer if one of X¹¹ and X¹² is a single bond or a group with a    length of two atoms

wherein one or more H atoms in —(CH₂)_(n)— may independently of eachother optionally be replaced by F or CH₃.

In a further preferred embodiment, which may be different from oridentical with any of the preferred embodiments,

-   X¹¹ and X¹² both are a single bond and-   Sp¹ is —(CH₂)_(n)— with-   n 1, 3 or an integer from 5 to 15, most preferably an odd (i.e.    uneven) integer and, most preferably 5, 7 or 9,

wherein one or more H atoms in —(CH₂)_(n)— may independently of eachother optionally be replaced by F or CH₃.

Preferred compounds of formula I are compounds in which

-   X¹¹-Sp¹-X¹²— is -Sp¹-, -Sp¹-O—, —O-Sp¹-, —O-Sp¹-O—, -Sp¹-CO—O—,    —O—CO-Sp¹-, —O-Sp¹-CO—O—, —O—CO-Sp¹-O—, or —O—CO-Sp¹-CO—O—, CO—O—,    preferably -Sp¹-, -Sp¹-O—, —O-Sp¹-, —O-Sp¹-O—, -Sp¹-CO—O—,    —O—CO-Sp¹- or —O—CO-Sp¹-CO—O—, more preferably -Sp¹-, —O-Sp¹-O—,    -Sp¹-CO—O— or —O—CO-Sp¹-CO—O—,-   Sp¹ is —(CH₂)_(n)— with-   n 1, 3 or an integer from 5 to 15, most preferably an odd (i.e.    uneven) integer and, most preferably 5, 7 or 9,

wherein one or more H atoms in —(CH₂)_(n)— may independently of eachother optionally be replaced by F or CH₃.

Further preferred compounds of formula I are compounds in which

-   MG¹¹ and MG¹² are independently from one another a group of    (partial) formula II

-A¹¹-(Z¹¹-A¹²)_(k)-  II

wherein

-   Z¹¹ are, independently of each other in each occurrence, a single    bond, —COO—, —OCO—, —O—CO—O—, —OCH₂—, —CH₂O—, —OCF₂—, —CF₂O—,    —CH₂CH₂—, —(CH₂)₄—, —CF₂CF₂—, —CH═CH—, —CF═CF—, —CH═CH—COO—,    —OCO—CH═CH— or —C≡C—, optionally substituted with one or more of F,    S and/or Si, preferably a single bond,-   one or more of-   A¹¹ and A¹² are each independently in each occurrence a comprise a    5-atomic ring, and are preferably selected from thiophene-2,5-diyl,    furane-2,5-diyl, thiazole-diyl, thiadiazole-diyl, it being possible    for all these groups to be unsubstituted, mono-, di-, tri- or    tetrasubstituted with F, Cl, CN or alkyl, alkoxy, alkylcarbonyl or    alkoxycarbonyl groups with 1 to 7 C atoms, wherein one or more H    atoms may be substituted by F or Cl, preferably F, Cl, CH₃ or CF₃,    and the other,-   A¹¹ and A¹² are each independently in each occurrence 1,4-phenylene,    wherein in addition one or more CH groups may be replaced by N,    trans-1,4-cyclo-hexylene in which, in addition, one or two    non-adjacent CH₂ groups may be replaced by O and/or S,    1,4-cyclohexenylene, 1,4-bicyclo-(2,2,2)-octylene,    piperidine-1,4-diyl, naphthalene-2,6-diyl,    decahydro-naphthalene-2,6-diyl,    1,2,3,4-tetrahydro-naphthalene-2,6-diyl, cyclobutane-1,3-diyl,    spiro[3.3]heptane-2,6-diyl or dispiro[3.1.3.1]decane-2,8-diyl, it    being possible for all these groups to be unsubstituted, mono-, di-,    tri- or tetrasubstituted with F, Cl, CN or alkyl, alkoxy,    alkylcarbonyl or alkoxycarbonyl groups with 1 to 7 C atoms, wherein    one or more H atoms may be substituted by F or Cl, preferably F, Cl,    CH₃ or CF₃, and-   k is 0, 1, 2, 3 or 4, preferably 1, 2 or 3 and, most preferably 1 or    2.

A smaller group of preferred mesogenic groups of formula II comprisingonly 6-membered rings is listed below. For reasons of simplicity, Phe inthese groups is 1,4-phenylene, PheL is a 1,4-phenylene group which issubstituted by 1 to 4 groups L, with L being preferably F, Cl, CN, OH,NO₂ or an optionally fluorinated alkyl, alkoxy or alkanoyl group with 1to 7 C atoms, very preferably F, Cl, CN, OH, NO₂, CH₃, C₂H₅, OCH₃,OC₂H₅, COCH₃, COC₂H₅, COOCH₃, COOC₂H₅, CF₃, OCF₃, OCHF₂, OC₂F₅, inparticular F, Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃ and OCF₃, most preferablyF, Cl, CH₃, OCH₃ and COCH₃ and Cyc is 1,4-cyclohexylene. This listcomprises the subformulae shown below as well as their mirror images

-Phe-Z-Phe-  II-1

-Phe-Z-Cyc-  II-2

-Cyc-Z-Cyc-  II-3

-Phe-Z-PheL-  II-4

-PheL-Z-Phe-  II-5

-PheL-Z-Cyc-  II-6

-PheL-Z-PheL-  II-7

-Phe-Z-Phe-Z-Phe-  II-8

-Phe-Z-Phe-Z-Cyc-  II-9

-Phe-Z-Cyc-Z-Phe-  II-10

-Cyc-Z-Phe-Z-Cyc-  II-11

-Phe-Z-Cyc-Z-Cyc-  II-12

-Cyc-Z-Cyc-Z-Cyc-  II-13

-Phe-Z-Phe-Z-PheL-  II-14

-Phe-Z-PheL-Z-Phe-  II-15

-PheL-Z-Phe-Z-Phe-  II-16

-PheL-Z-Phe-Z-PheL-  II-17

-PheL-Z-PheL-Z-Phe-  II-18

-PheL-Z-PheL-Z-PheL-  II-19

-Phe-Z-PheL-Z-Cyc-  II-29

-Phe-Z-Cyc-Z-PheL-  II-21

-Cyc-Z-Phe-Z-PheL-  II-22

-PheL-Z-Cyc-Z-PheL-  II-23

-PheL-Z-PheL-Z-Cyc-  II-24

-PheL-Z-Cyc-Z-Cyc-  II-25

-Cyc-Z-PheL-Z-Cyc-  II-26

wherein

-   Cyc is 1,4-cyclohexlene, preferably trans-1,4-cyclohexlene,-   Phe is 1,4-phenylene,-   PheL is 1,4-phenylene, which is substituted by one, two or three    fluorine atoms, by one or two Cl atoms or by one Cl atom and one F    atom, and-   Z has one of the meanings of Z¹¹ as given under partial formula II,    at least one is preferably selected from —COO—, —OCO—, —O—CO—O—,    —OCH₂—, —CH₂O—, —OCF₂— or —CF₂O—.

Particularly preferred are the sub-formulae II-1, II-4, II-5, II-7,II-8, II-14, II-15, II-16, II-17, II-18 and II-19.

In these preferred groups Z in each case independently has one of themeanings of Z¹¹ as given under formula I. Preferably one of Z is —COO—,—OCO—, —CH₂—O—, —O—CH₂—, —CF₂—O— or —O—CF₂—, more preferably —COO—,—O—CH₂— or —CF₂—O—, and the others preferably are a single bond.

Very preferably, at least one of the mesogenic groups MG¹¹ and MG¹² is,and preferably, both of them are each and independently, selected fromthe following formulae IIa to IIn (the two reference Nos. “II i” and “III” being deliberately omitted to avoid any confusion) and their mirrorimages

wherein

L is in each occurrence independently of each other F or Cl, preferablyF and

r is in each occurrence independently of each other 0, 1, 2 or 3,preferably 0, 1 or 2.

The group

in these preferred formulae is very preferably denoting

furthermore

L is in each occurrence independently of each other F or Cl, F.

In case of compounds with an unpolar group, R¹¹ and R¹² are preferablyalkyl with up to 10 C atoms or alkoxy with 2 to 10 C atoms.

If R¹¹ or R¹² is an alkyl or alkoxy radical, i.e. where the terminal CH₂group is replaced by —O—, this may be straight chain or branched. It ispreferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms andaccordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy or octoxy,furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy ortetradecoxy, for example.

Oxaalkyl, i.e. where one CH₂ group is replaced by —O—, is preferablystraight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-,7-, 8- or 9-oxadecyl, for example.

In case of a compounds with a terminal polar group, R¹¹ and R¹² areselected from CN, NO₂, halogen, OCH₃, OCN, SCN, COR^(x), COOR^(x) or amono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 Catoms. R^(x) is optionally fluorinated alkyl with 1 to 4, preferably 1to 3 C atoms. Halogen is preferably F or Cl.

Especially preferably R¹¹ and R¹² in formula I are selected of H, F, Cl,CN, NO₂, OCH₃, COCH₃, COC₂H₅, COOCH₃, COOC₂H₅, CF₃, C₂F₅, in particularof H, F, Cl, CN, especially of H, F, and CN.

In addition, compounds of formula I containing an achiral branched groupR¹¹ and/or R¹² may occasionally be of importance, for example, due to areduction in the tendency towards crystallisation. Branched groups ofthis type generally do not contain more than one chain branch. Preferredachiral branched groups are isopropyl, isobutyl (=methylpropyl),isopentyl (=3-methylbutyl), isopropoxy, 2-methyl-propoxy and3-methylbutoxy.

The spacer group Sp¹ is preferably a linear or branched alkylene grouphaving 1, 3 or 5 to 40 C atoms, in particular 1, 3 or 5 to 25 C atoms,very preferably 1, 3 or 5 to 15 C atoms, and most preferably 5 to 15 Catoms, in which, in addition, one or more non-adjacent and non-terminalCH₂ groups may be replaced by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—,—S—CO—, —O—COO—, —CO—S—, —CO—O—, —CH(halogen)-, —CH(CN)—, —CH═CH— or—C≡C—. “Terminal” CH₂ groups are those directly bonded to the mesogenicgroups. Accordingly, “non-terminal” CH₂ groups are not directly bondedto the mesogenic groups MG¹¹ and MG¹².

Typical spacer groups are for example —(CH₂)_(o)—,—(CH₂CH₂O)_(p)—CH₂CH₂—, with o being an integer from 5 to 40, inparticular from 5 to 25, very preferably from 5 to 15, and p being aninteger from 1 to 8, in particular 1, 2, 3, 4, or 5.

Preferred spacer groups are pentylene, hexylene, heptylene, octylene,nonylene, decylene, undecylene, dodecylene, octadecylene,diethyleneoxyethylene, dimethyleneoxybutylene, pentenylene, heptenylene,nonenylene and undecenylene, for example.

Especially preferred are inventive compounds of formula I wherein Sp isdenoting alkylene with 5 to 15 C atoms. Straight-chain alkylene groupsare especially preferred.

Preferred are spacer groups, which are straight-chain alkylene with oddnumbers of C atoms, preferably a having 5, 7, 9, 11, 13 or 15 C atoms,very preferred are straight-chain alkylene spacers having 7, 9, and 11 Catoms.

In another embodiment of the present invention, the spacer groups arestraight-chain alkylenes with even numbers of C atoms, preferably having6, 8, 10, 12 and 14 C atoms. This embodiment is particularly preferredif one of X¹ and X¹² consists of one atom, i.e. is —S— or —O—, or ofthree atoms, e.g. is —S—CO—, —S—CO—S— or —S—CS—S—, and the other doesnot consist of one or three C atoms.

Especially preferred are inventive compounds of formula I wherein Sp isdenoting complete deuterated alkylene with 5 to 15 C atoms. Verypreferred are deuterated straight-chain alkylene groups. Most preferredare partially deuterated straight-chain alkylene groups.

Preferred are compounds of formula I wherein the mesogenic groupsR¹¹-MG¹¹-X¹¹— and R¹²-MG¹²-X¹²— are different from each other. Inanother embodiment compounds of formula I wherein R¹¹-MG¹¹-X¹¹— andR¹²-MG¹²-X¹²— in formula I are identical to each other.

Preferred compounds of formula I are selected from the group ofcompounds of formulae IA to IQ,

wherein the alkylene spacers (—CH₂)_(n)— shown are exemplary only and ntherein is an integer of 3 or from 5 to 15, preferably 5, 7 or 9, moreas shown,

R¹¹ and R¹² are independently from each other as defined above,including the preferred meanings of these groups, preferably R¹¹ is F orCN, preferably R¹² is CF₃, F or CN, more preferably F or CN and mostpreferably CN and wherein L is in each occurrence independently of eachother F, Cl or preferably F or Cl, most preferably F.

Particularly preferred compounds are selected from the group of formulaegiven above, which bear 0, 2 or 4 F atoms in lateral positions (i.e. asL).

The compounds of formula I can be synthesized according to or in analogyto methods which are known per se and which are described in standardworks of organic chemistry such as, for example, Houben-Weyl, Methodender organischen Chemie, Thieme-Verlag, Stuttgart. A preferred method ofpreparation can be taken from the following synthesis schemes.

Compounds of formula I, when added to a nematic liquid crystallinemixture, producing a phase below the nematic. In this context, a firstindication of the influence of bimesogenic compounds on nematic liquidcrystal mixtures was reported by Barnes, P. J., Douglas, A. G., Heeks,S. K., Luckhurst, G. R., Liquid Crystals, 1993, Vol. 13, No. 4, 603-613.This reference exemplifies highly polar alkyl spacered dimers andperceives a phase below the nematic, concluding it is a type of smectic.

A photo evidence of an existing mesophase below the nematic phase waspublished by Henderson, P. A., Niemeyer, O., Imrie, C. T. in LiquidCrystals, 2001, Vol. 28, No. 3, 463-472, which was not furtherinvestigated.

In Liquid Crystals, 2005, Vol. 32, No. 11-12, 1499-1513 Henderson, P.A., Seddon, J. M. and Imrie, C. T. reported, that the new phase belowthe nematic belonged in some special examples to a smectic C phase. Aadditional nematic phase below the first nematic was reported by Panov,V. P., Ngaraj, M., Vij, J. K., Panarin, Y. P., Kohlmeier, A., Tamba, M.G., Lewis, R. A. and Mehl, G. H. in Phys. Rev. Lett. 2010, 105,1678011-1678014.

In this context, liquid crystal mixtures comprising bimesogeniccompounds of formula I may show also a novel mesophase that is beingassigned as a second nematic phase. This mesophase exists at a lowertemperature than the original nematic liquid crystalline phase and hasbeen observed in the unique mixture concepts presented by thisapplication.

Accordingly, the bimesogenic compounds of formula I allow the secondnematic phase to be induced in nematic mixtures that do not have thisphase normally. Furthermore, varying the amounts of compounds of formulaI allow the phase behaviour of the second nematic to be tailored to therequired temperature.

Some preferred embodiments of the mixtures according to the inventionare indicated below.

Preferred are compounds of formula I wherein the mesogenic groups MG¹¹and MG¹² at each occurrence independently from each other comprise one,two or three six-membered rings, preferably two or three six-memberedrings.

The media according to the invention preferably comprise one, two,three, four or more, preferably one, two or three, compounds of theformula I.

The amount of compounds of formula I in the liquid crystalline medium ispreferably from 60 to 100%, in particular from 70 to 95%, verypreferably 80 to 90% by weight of the total mixture. In anotherpreferred embodiment, the medium according to the present inventionconsists of two or more, preferably three or more, more preferably fouror more compounds of formula I.

In a preferred embodiment the liquid crystalline medium according to thepresent invention optionally comprises one or more compounds of formulaIII, like those or similar to those known from GB 2 356 629.

R³¹-MG³¹-X³¹-Sp³-X³²-MG³²-R³²  III

wherein

-   R³¹ and R³² are each independently H, F, Cl, CN, NCS or a    straight-chain or branched alkyl group with 1 to 25 C atoms which    may be unsubstituted, mono- or polysubstituted by halogen or CN, it    being also possible for one or more non-adjacent CH₂ groups to be    replaced, in each case independently from one another, by —O—, —S—,    —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —O—CO—O—, —S—CO—, —CO—S—,    —CH═CH—, —CH═CF—, —CF═CF— or —C≡C— in such a manner that oxygen    atoms are not linked directly to one another,-   MG³¹ and MG³² are each independently a mesogenic group,-   Sp³ is a spacer group comprising 5 to 40 C atoms, wherein one or    more non-adjacent CH₂ groups may also be replaced by —O—, —S—, —NH—,    —N(CH₃)—, —CO—, —O—CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O—,    —CH(halogen)-, —CH(CN)—, —CH═CH— or —C≡C—, and-   X³¹ and X³² are each independently —O—, —S—, —CO—, —COO—, —OCO—,    —O—CO—O—, —CO—NH—, —NH—CO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —SCH₂—,    —CH₂S—, —CH═CH—, —CH═CH—COO—, —OCO—CH═CH—, —C≡C— or a single bond,    and

with the condition that compounds of formula I are excluded.

The mesogenic groups MG³¹ and MG³² are preferably selected of formulaII.

Especially preferred are compounds of formula III wherein R³¹-MG³¹-X³¹-and R³²-MG³²-X³²— are identical.

Another preferred embodiment of the present invention relates tocompounds of formula III wherein R³¹-MG³¹-X³¹— and R³²-MG³²-X³²— aredifferent.

Especially preferred are compounds of formula III wherein the mesogenicgroups MG³¹ and MG³² comprise one, two or three six-membered rings verypreferably are the mesogenic groups selected from formula II as listedbelow.

For MG³¹ and MG³² in formula III are particularly preferred are thesubformulae II-1, II-4, II-6, II-7, II-13, II-14, II-15, II-16, II-17and II-18. In these preferred groups Z in each case independently hasone of the meanings of Z¹ as given in formula II. Preferably Z is —COO—,—OCO—, —CH₂CH₂—, —C≡C— or a single bond.

Very preferably the mesogenic groups MG³¹ and MG³² are selected from theformulae IIa to IIo and their mirror images.

In case of compounds with a unon-polar group, R³¹ and R³² are preferablyalkyl with up to 15 C atoms or alkoxy with 2 to 15 C atoms.

If R³¹ or R³² is an alkyl or alkoxy radical, i.e. where the terminal CH₂group is replaced by —O—, this may be straight-chain or branched. It ispreferably straight-chain, has 2, 3, 4, 5, 6, 7 or 8 carbon atoms andaccordingly is preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl,octyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, or octoxy,furthermore methyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, nonoxy, decoxy, undecoxy, dodecoxy, tridecoxy ortetradecoxy, for example.

Oxaalkyl, i.e. where one CH₂ group is replaced by —O—, is preferablystraight-chain 2-oxapropyl (=methoxymethyl), 2- (=ethoxymethyl) or3-oxabutyl (=2-methoxyethyl), 2-, 3-, or 4-oxapentyl, 2-, 3-, 4-, or5-oxahexyl, 2-, 3-, 4-, 5-, or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl or 2-, 3-, 4-, 5-, 6-,7-, 8- or 9-oxadecyl, for example.

In case of a compounds with a terminal polar group, R³¹ and R³² areselected from CN, NO₂, halogen, OCH₃, OCN, SCN, COR^(x), COOR^(x) or amono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 Catoms. R^(x) is optionally fluorinated alkyl with 1 to 4, preferably 1to 3 C atoms.

Halogen is preferably F or Cl.

Especially preferably R³¹ and R³² in formula III are selected of F, Cl,CN, NO₂, OCH₃, COCH₃, COC₂H₅, COOCH₃, COOC₂H₅, CF₃, C₂F₅, OCF₃, OCHF₂,and OC₂F₅, in particular of F, Cl, CN, OCH₃ and OCF₃.

As for the spacer group Sp³ in formula III all groups can be used thatare known for this purpose to the skilled in the art. The spacer groupSp is preferably a linear or branched alkylene group having 5 to 40 Catoms, in particular 5 to 25 C atoms, very preferably 5 to 15 C atoms,in which, in addition, one or more non-adjacent CH₂ groups may bereplaced by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—, —S—CO—, —O—COO—,—CO—S—, —CO—O—, —CH(halogen)-, —CH(CN)—, —CH═CH— or —C≡C—.

Typical spacer groups are for example —(CH₂)_(o)—,—(CH₂CH₂O)_(p)—CH₂CH₂—, —CH₂CH₂—S—CH₂CH₂— or —CH₂CH₂—NH—CH₂CH₂—, with obeing an integer from 5 to 40, in particular from 5 to 25, verypreferably from 5 to 15, and p being an integer from 1 to 8, inparticular 1, 2, 3 or 4.

Preferred spacer groups are pentylene, hexylene, heptylene, octylene,nonylene, decylene, undecylene, dodecylene, octadecylene,diethyleneoxyethylene, dimethyleneoxybutylene, pentenylene, heptenylene,nonenylene and undecenylene, for example.

Especially preferred are inventive compounds of formula III wherein Sp³is denoting alkylene with 5 to 15 C atoms. Straight-chain alkylenegroups are especially preferred.

In another preferred embodiment of the invention the chiral compounds offormula III comprise at least one spacer group Sp¹ that is a chiralgroup of the formula IV.

X³¹ and X³² in formula III denote preferably —O—, —CO—, —COO—, —OCO—,—O—CO—O— or a single bond. Particularly preferred are the followingcompounds selected from formulae III-1 to III-4:

wherein R³¹, R³² have the meaning given under formula Ill, Z³¹ andZ^(31-I) are defined as Z³¹ and Z³² and Z^(32-I) are respectively thereverse groups of Z³¹ and Z^(32-I) in formula III and o and r areindependently at each occurrence as defined above, including thepreferred meanings of these groups and wherein L is in each occurrenceindependently of each other preferably F, Cl, CN, OH, NO₂ or anoptionally fluorinated alkyl, alkoxy or alkanoyl group with 1 to 7 Catoms, very preferably F, Cl, CN, OH, NO₂, CH₃, C₂H₅, OCH₃, OC₂H₅,COCH₃, COC₂H₅, COOCH₃, COOC₂H₅, CF₃, OCF₃, OCHF₂, OC₂F₅, in particularF, Cl, CN, CH₃, C₂H₅, OCH₃, COCH₃ and OCF₃, most preferably F, Cl, CH₃,OCH₃ and COCH₃ and from which compounds of formula I are excluded.

Particularly preferred mixtures according to the invention comprise oneor more compounds of the formulae III-1a to III-1e and III-3a to III-3b.

wherein the parameters are as defined above.

In a preferred embodiment of the invention the liquid crystalline mediumcomprises 2 to 25, preferably 3 to 15 compounds of formula Ill.

The total amount of compounds of formula Ill in the liquid crystallinemedium is preferably from 0 to 50%, in particular from 2 to 30%, verypreferably 3 to 15% by weight of the total mixture.

In another preferred embodiment, the medium according to the presentinvention consists of two or more, preferably three or more, morepreferably four or more compounds of formula I and of one or morecompounds of formula Ill.

Particularly preferred media according to the invention optionallycomprise one or more chiral dopants which themselves do not necessarilyhave to show a liquid crystalline phase and give good uniform alignmentthemselves.

Especially preferred are chiral dopants selected from formula IV

and formula V

including the respective (S,S) enantiomer,

wherein E and F are each independently 1,4-phenylene ortrans-1,4-cyclo-hexylene, v is 0 or 1, Z⁰ is —COO—, —OCO—, —CH₂CH₂— or asingle bond, and R is alkyl, alkoxy or alkanoyl with 1 to 12 C atoms.

The compounds of formula IV and their synthesis are described in WO98/00428. Especially preferred is the compound CD-1, as shown in table Dbelow. The compounds of formula V and their synthesis are described inGB 2,328,207.

Especially preferred are chiral dopants with a high helical twistingpower (HTP), in particular those disclosed in WO 98/00428.

Further typically used chiral dopants are e.g. the commerciallyavailable R/S-5011, CD-1, R/S-811 and CB-15 (from Merck KGaA, Darmstadt,Germany).

The above mentioned chiral compounds R/S-5011 and CD-1 and the compoundsof formula IV and V exhibit a very high helical twisting power (HTP),and are therefore particularly useful for the purpose of the presentinvention.

The liquid crystalline medium preferably comprises preferably 1 to 5, inparticular 1 to 3, very preferably 1 or 2 chiral dopants, preferablyselected from the above formula IV, in particular CD-1, and/or formula Vand/or R-5011 or S-5011, very preferably, the chiral compound is R-5011,S-5011 or CD-1.

The amount of chiral compounds in the liquid crystalline medium ispreferably from 1 to 15%, in particular from 0.5 to 10%, very preferably0.1 to 5% by weight of the total mixture.

In another preferred embodiment, the medium according to the presentinvention consists of two or more, preferably three or more, morepreferably four or more compounds of formula I and one or more chiraldopants.

Further preferred are liquid crystalline media optionally comprising oneor more additives selected from the following formula VI

wherein

R⁵ is alkyl, alkoxy, alkenyl or alkenyloxy with up to 12 C atoms,

L¹ through L⁴ are each independently H or F,

Z² is —COO—, —CH₂CH₂— or a single bond,

m is 1 or 2

Particularly preferred compounds of formula VI are selected from thefollowing formulae

wherein, R has one of the meanings of R⁵ above and L¹, L² and L³ havethe above meanings.

The liquid crystalline medium preferably comprises preferably 1 to 5, inparticular 1 to 3, very preferably 1 or 2, preferably selected from theabove formulae VIa to VIf, very preferably from formulae VIf.

The amount of suitable additives of formula VI in the liquid crystallinemedium is preferably from 1 to 20%, in particular from 1 to 15%, verypreferably 1 to 10% by weight of the total mixture.

The liquid crystal media according to the present invention may containfurther additives in usual concentrations. The total concentration ofthese further constituents is in the range of 0.1% to 10%, preferably0.1% to 6%, based on the total mixture. The concentrations of theindividual compounds used each are preferably in the range of 0.1% to3%.

The concentration of these and of similar additives is not taken intoconsideration for the values and ranges of the concentrations of theliquid crystal components and compounds of the liquid crystal media inthis application. This also holds for the concentration of the dichroicdyes used in the mixtures, which are not counted when the concentrationsof the compounds respectively the components of the host medium arespecified. The concentration of the respective additives is always givenrelative to the final doped mixture.

The liquid crystal media according to the present invention consists ofseveral compounds, preferably of 2 to 30, more preferably of 3 to 20 andmost preferably of 4 to 16 compounds. These compounds are mixed inconventional way. As a rule, the required amount of the compound used inthe smaller amount is dissolved in the compound used in the greateramount. In case the temperature is above the clearing point of thecompound used in the higher concentration, it is particularly easy toobserve completion of the process of dissolution. It is, however, alsopossible to prepare the media by other conventional ways, e.g. using socalled pre-mixtures, which can be e.g. homologous or eutectic mixturesof compounds or using so called multi-bottle-systems, the constituentsof which are ready to use mixtures themselves.

The liquid crystalline media comprising at least 60% of one or morecompounds of formula I can be used in liquid crystal displays, such asSTN, TN, AMD-TN, temperature compensation, guest-host, phase change orsurface stabilized or polymer stabilized cholesteric texture (SSCT,PSCT) displays, in particular in flexoelectric devices, in active andpassive optical elements like polarizers, compensators, reflectors,alignment layers, colour filters or holographic elements, in adhesives,synthetic resins with anisotropic mechanical properties, cosmetics,diagnostics, liquid crystal pigments, for decorative and securityapplications, in nonlinear optics, optical information storage or aschiral dopants.

The mixtures in accordance with the present invention are particularlyuseful for flexoelectric liquid crystal display. Thus, another object ofthe present invention is a flexoelectric display comprising a liquidcrystal medium comprising at least 60% of one or more compounds offormula I.

In a preferred embodiment, the mixtures of the present invention can bealigned planar. Planar alignment can be achieved e.g. by means of analignment layer, for example a layer of rubbed polyimide or sputteredSiO_(x), that is applied on top of the substrate.

Alternatively, it is possible to directly rub the substrate, i.e.without applying an additional alignment layer. For example, rubbing canbe achieved by means of a rubbing cloth, such as a velvet cloth, or witha flat bar coated with a rubbing cloth. In a preferred embodiment of thepresent invention rubbing is achieved by means of a at least one rubbingroller, like e.g. a fast spinning roller that is brushing across thesubstrate, or by putting the substrate between at least two rollers,wherein in each case at least one of the rollers is optionally coveredwith a rubbing cloth. In another preferred embodiment of the presentinvention rubbing is achieved by wrapping the substrate at leastpartially at a defined angle around a roller that is preferably coatedwith a rubbing cloth.

In a preferred embodiment, the mixtures in accordance with the presentinvention not comprising any chiral compounds can be homeotropic alignedby filling the mixture in the isotropic phase into cells comprisingcommercially VA PI alignment layers, such as Duramide 32 obtainable fromFujifilm, and cooling the mixture from the isotropic phase into thenematic phase.

For some preferred mixtures an isotropic to planar nematic transition isobserved. After further cooling of the cell in the nematic phase thetexture viewed between crossed polarisers is observed to change fromplanar (uniformly coloured) to homeotropic (black). This state isconfirmed as being a homeotropic nematic texture due to ‘pressing’ thecell and observing a tell-tale bright flash.

However, it is further preferred that the mixture of the presentinvention exhibit a direct isotropic to homeotropic nematic transition.

Upon further cooling the mixture remains homeotropic nematic down toroom temperature and below.

Further suitable methods to achieve planar or homeotropic alignment aredescribed for example in J. Cognard, Mol. Cryst. Liq. Cryst. 78,Supplement 1, 1-77 (1981).

In another preferred embodiment, the mixtures of the present inventioncan be aligned in their cholesteric phase.

The switching between different states of orientation according to apreferred embodiment of the present invention is exemplarily describedbelow in detail for a sample of an inventive compound of formula I.

According to this preferred embodiment, the sample is placed into a cellcomprising two plane-parallel glass plates coated with electrode layers,e.g. ITO layers, and aligned in its cholesteric phase into a planarstate wherein the axis of the cholesteric helix is oriented normal tothe cell walls. This state is also known as Grandjean state, and thetexture of the sample, which is observable e.g. in a polarizationmicroscope, as Grandjean texture. Planar alignment can be achieved e.g.by surface treatment of the cell walls, for example by rubbing and/orcoating with an alignment layer such as polyimide.

A Grandjean state with a high quality of alignment and only few defectscan further be achieved by heating the sample to the isotropic phase,subsequently cooling to the chiral nematic phase at a temperature closeto the chiral nematic-isotropic phase transition, and flow alignment bylightly pressing the cell.

In the planar state, the sample shows selective reflection of incidentlight, with the central wavelength of reflection depending on thehelical pitch and the mean refractive index of the material.

When an electric field is applied to the electrodes, for example with afrequency from 10 Hz to 1 kHz, and an amplitude of up to 12 V_(rms)/μm,the sample is being switched into a homeotropic state where the helix isunwound and the molecules are oriented parallel to the field, i.e.normal to the plane of the electrodes. In the homeotropic state, thesample is transmissive when viewed in normal daylight, and appears blackwhen being put between crossed polarizers.

Upon reduction or removal of the electric field in the homeotropicstate, the sample adopts a focal conic texture, where the moleculesexhibit a helically twisted structure with the helical axis beingoriented perpendicular to the field, i.e. parallel to the plane of theelectrodes. A focal conic state can also be achieved by applying only aweak electric field to a sample in its planar state. In the focal conicstate the sample is scattering when viewed in normal daylight andappears bright between crossed polarizers.

A sample of a medium in accordance with the present invention indifferent states of orientation exhibits different transmission oflight. Therefore, the respective state of orientation, as well as itsquality of alignment, can be controlled by measuring the lighttransmission of the sample depending on the strength of the appliedelectric field. Thereby it is also possible to determine the electricfield strength required to achieve specific states of orientation andtransitions between these different states. In a sample of a medium inaccordance with the present invention, the above described focal conicstate consists of many disordered birefringent small domains. Byapplying an electric field greater than the field for nucleation of thefocal conic texture, preferably with additional shearing of the cell, auniformly aligned texture is achieved where the helical axis is parallelto the plane of the electrodes in large, well-aligned areas. Inaccordance with the literature on state of the art chiral nematicmaterials, such as P. Rudquist et al., Liq. Cryst. 23 (4), 503 (1997),this texture is also called uniformly-lying helix (ULH) texture. Thistexture is required to characterize the flexoelectric properties of theinventive compound.

In a preferred embodiment of the present invention, the sequence oftextures typically observed in a sample having a Δε>0 on a rubbedpolyimide substrate upon increasing or decreasing electric field isgiven below:

Starting from the ULH texture, the inventive mixtures can be subjectedto flexoelectric switching by application of an electric field. Thiscauses rotation of the optic axis of the material in the plane of thecell substrates, which leads to a change in transmission when placingthe material between crossed polarizers. The flexoelectric switching ofinventive materials is further described in detail in the introductionabove and in the examples.

It is also possible to obtain the ULH texture, starting from the focalconic texture, by applying an electric field with a high frequency, offor example 10 kHz, to the sample whilst cooling slowly from theisotropic phase into the cholesteric phase and shearing the cell. Thefield frequency may differ for different compounds.

The media in accordance with the present invention are particularlyuseful in flexoelectric liquid crystal displays as they can easily bealigned into macroscopically uniform orientation, and exhibit highvalues of the elastic constant k₁₁ and a high flexoelectric coefficiente.

The liquid crystal medium preferably exhibits a k₁₁<1×10⁻¹⁰ N,preferably <2×10⁻¹¹ N, and a flexoelectric coefficient e>1×10⁻¹¹ C/m,preferably >1×10⁻¹⁰ C/m.

Apart from the use in flexoelectric devices, the mixtures in accordancewith the present invention are also suitable for other types of displaysand other optical and electro optical applications, such as opticalcompensation or polarizing films, colour filters, reflectivecholesterics, optical rotatory power and optical information storage.

A further aspect of the present invention relates to a display cellwherein the cell walls exhibit hybrid alignment conditions. The term“hybrid alignment” or orientation of a liquid crystal or mesogenicmaterial in a display cell or between two substrates means that themesogenic groups adjacent to the first cell wall or on the firstsubstrate exhibit homeotropic orientation and the mesogenic groupsadjacent to the second cell wall or on the second substrate exhibitplanar orientation.

The term “homeotropic alignment” or orientation of a liquid crystal ormesogenic material in a display cell or on a substrate means that themesogenic groups in the liquid crystal or mesogenic material areoriented substantially perpendicular to the plane of the cell orsubstrate, respectively.

The term “planar alignment” or orientation of a liquid crystal ormesogenic material in a display cell or on a substrate means that themesogenic groups in the liquid crystal or mesogenic material areoriented substantially parallel to the plane of the cell or substrate,respectively.

A flexoelectric display according to a preferred embodiment of thepresent invention comprises two plane parallel substrates, preferablyglass plates covered with a transparent conductive layer such as indiumtin oxide (ITO) on their inner surfaces, and a flexoelectric liquidcrystalline medium provided between the substrates, characterized inthat one of the inner substrate surfaces exhibits homeotropic alignmentconditions and the opposite inner substrate surface exhibits planaralignment conditions for the liquid crystalline medium.

By using a display cell with hybrid alignment conditions, a very highswitching angle of flexoelectric switching, fast response times and agood contrast can be achieved.

The flexoelectric display according to present invention may alsocomprise plastic substrates instead of glass substrates. Plastic filmsubstrates are particularly suitable for rubbing treatment by rubbingrollers as described above.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following examples are, therefore, to beconstrued as merely illustrative and not limitative of the remainder ofthe disclosure in any way whatsoever.

Unless the context clearly indicates otherwise, as used herein pluralforms of the terms herein are to be construed as including the singularform and vice versa.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, mean “including but not limited to”, andare not intended to (and do not) exclude other components.

Throughout the present application it is to be understood that theangles of the bonds at a C atom being bound to three adjacent atoms,e.g. in a C═C or C═O double bond or e.g. in a benzene ring, are 120° andthat the angles of the bonds at a C atom being bound to two adjacentatoms, e.g. in a C≡C or in a C≡N triple bond or in an allylic positionC═C═C are 180°, unless these angles are otherwise restricted, e.g. likebeing part of small rings, like 3-, 5- or 5-atomic rings,notwithstanding that in some instances in some structural formulae theseangles are not represented exactly.

It will be appreciated that variations to the foregoing embodiments ofthe invention can be made while still falling within the scope of theinvention. Each feature disclosed in this specification, unless statedotherwise, may be replaced by alternative features serving the same,equivalent or similar purpose. Thus, unless stated otherwise, eachfeature disclosed is one example only of a generic series of equivalentor similar features.

All of the features disclosed in this specification may be combined inany combination, except combinations where at least some of suchfeatures and/or steps are mutually exclusive. In particular, thepreferred features of the invention are applicable to all aspects of theinvention and may be used in any combination. Likewise, featuresdescribed in non-essential combinations may be used separately (not incombination).

The total concentration of all compounds in the media according to thisapplication is 100%. All concentrations are given in % w/w, unlessexplicitly stated otherwise.

In the foregoing and in the following examples, unless otherwiseindicated, all temperatures are set forth uncorrected in degrees Celsiusand all parts and percentages are by weight.

The following abbreviations are used to illustrate the liquidcrystalline phase behavior of the compounds: K=crystalline; N=nematic;N2=second nematic; S or Sm=smectic; Ch=cholesteric; I=isotropic;Tg=glass transition. The numbers between the symbols indicate the phasetransition temperatures in ° C.

In the present application and especially in the following examples, thestructures of the liquid crystal compounds are represented byabbreviations, which are also called “acronyms”. The transformation ofthe abbreviations into the corresponding structures is straight forwardaccording to the following three tables A to C.

All groups C_(n)H_(2n+1), C_(m)H_(2m+1), and C_(l)H2_(l+1) arepreferably straight chain alkyl groups with n, m and l C-atoms,respectively, all groups C_(n)H_(2n), C_(m)H_(2m) and C_(l)H_(2l) arepreferably (CH₂)_(n), (CH₂)_(m) and (CH₂)_(l), respectively and —CH═CH—preferably is trans- respectively Evinylene.

Table A lists the symbols used for the ring elements, table B those forthe linking groups and table C those for the symbols for the left handand the right hand end groups of the molecules.

Table D lists exemplary molecular structures together with theirrespective codes.

TABLE A Ring Elements C

P

D

DI

A

AI

G

GI

G(CI)

GI(CI)

G(1)

GI(1)

U

UI

Y

M

MI

N

NI

np

n3f

n3fI

th

thI

th2f

th2fI

o2f

o2fI

dh

K

KI

L

LI

F

FI

TABLE B Linking Groups n (—CH₂—)_(n) “n” is an integer except 0 and 2 E—CH₂—CH₂— V —CH═CH— T —C≡C— W —CF₂—CF₂— B —CF═CF— Z —CO—O— ZI —O—CO— X—CF═CH— XI —CH═CF— 1O —CH₂—O— O1 —O—CH₂— Q —CF₂—O— QI —O—CF₂—

TABLE C End Groups Left hand side, used alone or in Right hand side,used alone or combination with others in combination with others -n-C_(n)H_(2n+1)— -n —C_(n)H_(2n+1) -nO— C_(n)H_(2n+1)—O— -nO—O—C_(n)H_(2n+1) —V— CH₂═CH— —V —CH═CH₂ -nV— C_(n)H_(2n+1)—CH═CH— -nV—C_(n)H_(2n)—CH═CH₂ —Vn- CH₂═CH—C_(n)H_(2n)— —Vn —CH═CH—C_(n)H_(2n+1)-nVm— C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m)— -nVm—C_(n)H_(2n)—CH═CH—C_(m)H_(2m+1) —N— N≡C— —N —C≡N —S— S═C═N— —S —N═C═S—F— F— —F —F —CL— Cl— —CL —Cl —M— CFH₂— —M —CFH₂ —D— CF₂H— —D —CF₂H —T—CF₃— —T —CF₃ —MO— CFH₂O— —OM —OCFH₂ —DO— CF₂HO— —OD —OCF₂H —TO— CF₃O——OT —OCF₃ —A— H—C≡C— —A —C≡C—H -nA— C_(n)H_(2n+1)—C≡C— —An—C≡C—C_(n)H_(2n+1) —NA— N≡C—C≡C— —AN —C≡C—C≡N Left hand side, used inRight hand side, used in combination with others only combination withothers only -...n...- (—CH₂—)_(n) -...n... (—CH₂—)_(n) —...M...— —CFH——...M... —CFH— —...D...— —CF₂— —...D... —CF₂— —...V...— —CH═CH— —...V...—CH═CH— —...Z...— —CO—O— —...Z... —CO—O— —...ZI...— —O—CO— —...ZI...—O—CO— —...K...— —CO— —...K... —CO— —...W...— —CF═CF— —...W... —CF═CF—

wherein n und m each are integers and three points “ . . . ” indicate aspace for other symbols of this table.

Preferably the liquid crystalline media according to the presentinvention comprise, besides the compound(s) of formula I one or morecompounds selected from the group of compounds of the formulae of thefollowing table.

TABLE D In this table n is an integer selected from 3 and 5 to 15,preferably from 3, 5, 7 and 9, unless explicitly defined otherwise.

wherein n is an integer from 12 t o 15, preferably an even integer.

Further preferred compounds of formula I according to the presentapplication are those abbreviated by the following acronyms:

wherein

m and k are independently of each other an integer from 1 to 9preferably from 1 to 7, more preferably from 3 to 5 and n is an integerfrom 1 to 15, preferably an odd integer from 3 to 9.

EXAMPLES

Typically a 5.6 μm thick cell, having an anti-parallel rubbed PIalignment layer, is filled on a hotplate at a temperature at which theflexoelectric mixture in the isotropic phase.

After the cell has been filled, the phase transitions, includingclearing point, are determined using Differential Scanning Calorimetry(DSC) and verified by optical inspection. For optical phase transitionmeasurements, a Mettler FP90 hot-stage controller connected to a FP82hot-stage is used to control the temperature of the cell. Thetemperature is increased from ambient temperature at a rate of 5 degreesC. per minute, until the onset of the isotropic phase is observed. Thetexture change is observed through crossed polarizers using an OlympusBX51 microscope and the respective temperature noted.

Wires are then attached to the ITO electrodes of the cell using indiummetal. The cell is secured in a Linkam THMS600 hot-stage connected to aLinkam TMS93 hot-stage controller. The hot-stage is secured to arotation stage in an Olympus BX51 microscope.

The cell is heated until the liquid crystal is completely isotropic. Thecell is then cooled under an applied electric field until the sample iscompletely nematic. The driving waveform is supplied by a TektronixAFG3021B arbitrary function generator, which is sent through aNewtons4th LPA400 power amplifier before being applied to the cell. Thecell response is monitored with a Thorlabs PDA55 photodiode. Both inputwaveforms and optical response are measured using a Tektronix TDS 2024Bdigital oscilloscope.

In order to measure the flexoelectric response of the material, thechange in the size of the tilt of the optic axis is measured as afunction of increasing voltage. This is achieved by using the equation:

${\tan \; \phi} = {\frac{P_{0}}{{2\pi}\;}\frac{e}{K}\underset{\_}{E}}$

wherein φ is the tilt in the optic axis from the original position (i.e.when E=0), E is the applied field, K is the elastic constant (average ofK₁ and K₃) and e is the flexoelectric coefficient (where e=e₁+e₃). Theapplied field is monitored using a HP 34401A multimeter. The tilt angleis measured using the aforementioned microscope and oscilloscope. Theundisturbed cholesteric pitch, P₀, is measured using an Ocean OpticsUSB4000 spectrometer attached to a computer. The selective reflectionband is obtained and the pitch determined from the spectral data.

The mixtures shown in the following examples are well suitable for usein ULH-displays. To that end an appropriate concentration of the chiraldopant or dopants used has to be applied in order to achieve a typicalcholesteric pitch of 350 to 275 nm.

Comparative Mixture Example C-0

The mixture C-0 is prepared and investigated in particular studying itsproperties for being aligned.

Composition Compound No. Abbreviation Conc./% 1 F-PGI-O-9-O-GP-F 25.0 2F-PGI-O-9-O-PP-N 25.0 3 F-PGI-ZI-9-Z-GP-F 25.0 4 F-PGI-ZI-9-Z-PP-N 25.0Σ 100.0

The alignment of the mixtures, like mixture C-0, is determined in a testcell with anti-parallel rubbed PI orientation layers, for planaralignment, having a cell gap of 10 μm at a wavelength of 550 nm. Theoptical retardation of the samples is determined using an elipsometerinstrument for various angles of incidence ranging from −60° to +40°.

The sample of C-0 shows an optical retardation of 25 nm underperpendicular observation (i.e. at an angle of incidence of 0°). Thisalready indicates the presence of a homogeneous alignment. For variousangles of incidence, the values of the retardation range from 2 nm to 55nm. Though they scatter quite significantly as a function of the angleof incidence, there seems to be a trend of the retardation increasingwith increasing angle of incidence. However, the significant scatter ofthe retardation values indicate a rather poor quality of the homeotropicalignment.

MIXTURE EXAMPLES

The following mixtures are prepared and investigated in particularstudying their properties for being aligned.

Mixture Example M-1

Composition Compound No. Abbreviation Conc./% T_(NI) (° C.) = 72 15-P-ZI-5-Z-GP-N 45 2 3-PY-ZI-7-Z-PP-N 35 3 3-CP-5-Z-PP-N 20 Σ 100

The mixture shows very good homeotropic alignment below 51° C. This isindicated by the retardation at normal incidence being close to zero andfurther the retardation increasing almost symmetrically for positive andnegative angles of incidence with increasing absolute value of the angleof incidence.

Mixture Example M-2

Composition Compound No. Abbreviation Conc./% T_(NI) (° C.) = 65 15-P-ZI-5-Z-GP-N 45 2 3-PY-ZI-7-Z-PP-N 35 3 5-P-ZI-5-Z-PP-N 20 Σ 100

The mixture shows very good homeotropic alignment below 54° C. This isindicated by the retardation at normal incidence being close to zero andfurther the retardation increasing almost symmetrically for positive andnegative angles of incidence with increasing absolute value of the angleof incidence.

Mixture Example M-3

Composition Compound No. Abbreviation Conc./% T_(NI) (° C.) = 58 15-P-ZI-5-Z-GP-N 45 2 5-P-ZI-5-Z-PP-N 27.5 3 3-CP-5-Z-PP-N 27.5 Σ 100

The mixture shows very good homeotropic alignment below 52° C. This isindicated by the retardation at normal incidence being close to zero andfurther the retardation increasing almost symmetrically for positive andnegative angles of incidence with increasing absolute value of the angleof incidence.

Mixture Example M-4

Composition Compound No. Abbreviation Conc./% T_(NI) (° C.) = 91 13-PY-ZI-7-Z-PP-N 40 2 5-P-ZI-5-Z-PP-N 30 3 3-CP-5-Z-PP-N 30 Σ 100

The mixture shows very good homeotropic alignment below 67° C. This isindicated by the retardation at normal incidence being close to zero andfurther the retardation increasing almost symmetrically for positive andnegative angles of incidence with increasing absolute value of the angleof incidence.

1. Medium comprising at least 60% of one or more compounds of formula I

wherein R¹¹ and R¹² are each independently H, F, Cl, CN, NCS or astraight-chain or branched alkyl group with 1 to 25 C atoms, which maybe unsubstituted, mono- or polysubstituted by halogen or CN, it beingalso possible for one or more non-adjacent CH₂ groups to be replaced, ineach occurrence independently from one another, by —O—, —S—, —NH—,—N(CH₃)—, —CO—, —COO—, —OCO—, —O—CO—O—, —S—CO—, —CO—S—, —CH═CH—,—CH═CF—, —CF═CF— or —C≡C— in such a manner that oxygen atoms are notlinked directly to one another, at least one of R¹¹ and R¹² is an alkylgroup, i.e. a straight-chain or branched alkyl group with 1 to 25 Catoms, which may be unsubstituted, mono- or polysubstituted by halogenor CN, it being also possible for one or more non-adjacent CH₂ groups tobe replaced, in each occurrence independently from one another, by —O—,—S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —O—CO—O—, —S—CO—, —CO—S—,—CH═CH—, —CH═CF—, —CF═CF— or —C≡C— in such a manner that oxygen atomsare not linked directly to one another, preferably a polar group, inwhich one CH₂ groups is replaced by —CH═CH—, —CH═CF—, —CF═CF—, but fromwhich OCF₃ and CF₃, are excluded, MG¹¹ and MG¹² are each independently amesogenic group, wherein at least one of MG¹¹ and MG¹² comprises one,two or more 5-atomic and/or 6-atomic rings, in case of comprising two ormore 5- and/or 6-atomic rings at least two of these may be linked by a2-atomic linking group, Sp¹ is a spacer group comprising 1, 3 or 5 to 40C atoms, wherein one or more non-adjacent and non-terminal CH₂ groupsmay also be replaced by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—, —S—CO—,—O—COO—, —CO—S—, —CO—O—, —CH(halogen)-, —CH(CN)—, —CH═CH— or —C≡C—,however in such a way that no two O-atoms are adjacent to one another,no two —CH═CH— groups are adjacent to each other and no two groupsselected from —O—CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O— and —CH═CH— areadjacent to each other, and X¹¹ and X¹² are independently from oneanother a linking group selected from —CO—O—, —O—CO—, —O—, —CH═CH—,—C≡C—, —CF₂—O—, —O—CF₂—, —CF₂—CF₂, —CH₂—O—, —O—CH₂—, —CO—S—, —S—CO—,—CS—S—, —S—, and a single bond, under the condition that in—X¹¹-Sp¹-X¹²— no two O atoms are adjacent to one another, no two —CH═CH—groups are adjacent to each other and no two groups selected from—O—CO—, —S—CO—, —O—COO—, —CO—S—, —CO—O— and —CH═CH— are adjacent to eachother.
 2. Medium according to claim 1, comprising one or more compoundsof formula I wherein at least one of MG¹¹ and MG¹² comprises one or two5-atomic rings, and one or more 6-atomic rings, and at least two ofthese are optionally linked by a 2-atomic group.
 3. Medium according toclaim 1, comprising one or more compounds of formula I wherein both MG¹¹and MG¹² comprise one or two 6-atomic rings.
 4. Medium according toclaim 1, comprising one or more compounds of formula I wherein R¹² isselected from F, Cl and CN.
 5. Medium according to claim 1, comprisingone or more compounds of formula I wherein Sp¹ is —(CH₂)_(o)— and o is1, 3 or an integer from 5 to
 15. 6. Medium according to claim 1,comprising one or more compounds of formula I wherein X¹¹ and X¹² areindependently from one another a linking group selected from —CO—O— anda single bond.
 7. Medium according to claim 1, characterised in that itoptionally comprises one or more compounds selected from the group ofthe compounds of the formulae IIIR³¹-MG³¹-X³¹-Sp³-X³²-MG³²-R³²  III wherein R³¹ and R³² are eachindependently H, F, Cl, CN, NCS or a straight-chain or branched alkylgroup with 1 to 25 C atoms which may be unsubstituted, mono- orpolysubstituted by halogen or CN, it being also possible for one or morenon-adjacent CH₂ groups to be replaced, in each case independently fromone another, by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —O—CO—O—,—S—CO—, —CO—S—, —CH═CH—, —CH═CF—, —CF═CF— or —C≡C— in such a manner thatoxygen atoms are not linked directly to one another, MG³¹ and MG³² areeach independently a mesogenic group, Sp³ is a spacer group comprising 5to 40 C atoms, wherein one or more non-adjacent CH₂ groups may also bereplaced by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —O—CO—, —S—CO—, —O—COO—,—CO—S—, —CO—O—, —CH(halogen)-, —CH(CN)—, —CH═CH— or —C≡C—, and X³¹ andX³² are each independently —O—, —S—, —CO—, —COO—, —OCO—, —O—CO—O—,—CO—NH—, —NH—CO—, —CH₂CH₂—, —OCH₂—, —CH₂O—, —SCH₂—, —CH₂S—, —CH═CH—,—CH═CH—COO—, —OCO—CH═CH—, —C≡C— or a single bond, with the conditionthat compounds of formula I are excluded.
 8. Medium according to claim1, consisting of two or more compounds of formula I.
 9. A method whichcomprises including a medium according to claim 1, in a liquid crystaldevice.
 10. Liquid crystal device comprising a liquid crystalline mediumcomprising a medium according to claim
 1. 11. Liquid crystal deviceaccording to claim 10, characterized in that it is a flexoelectricdevice.